In recent years development of a thinner and lighter-weighted note type personal computer having a larger image plane and a higher definition is in progress. Accordingly, a protective film for varieties of displays, specifically, a protective film for a liquid crystal display is also more and more intensively required to be thinner and wider, and to have higher quality. Generally, as polarizer protective film, a cellulose ester film is widely utilized. Cellulose ester film is generally wound around a core to form a film master roll, which is stored and transported.
Heretofore, this cellulose ester film has been primarily manufactured by a solution casting method. In a solution casting method, cellulose ester dissolved in a solvent is cast on a support to form a film, followed by evaporating the solvent to dry the film, and thus a film is obtained. Since the film obtained by a solution casting method has excellent flatness, a liquid crystal display exhibiting a high image quality without unevenness can be obtained by using the cellulose ester film.
However, a solution casting method requires a large amount of an organic solvent, which has been a problem in view of the large environmental load. Since cellulose ester film is cast by use of a halogen-containing solvent because of the excellent solubility characteristics, decrease of the using amount of a solvent is particularly required. Accordingly, it has become difficult to produce a larger amount of cellulose ester film by a solution casting method.
Further, since it is necessary to remove a solvent remaining in the film interior, facility investment to the manufacturing line such as a drying line, a drying energy and apparatuses for recover and regeneration of an evaporated solvent; and a manufacturing cost have been enormous, and reduction thereof is also an important subject.
In recent years, attempts to melt cast a cellulose ester have been made, for application in silver halide photography or as a polarizer protective film. However, since cellulose ester is a polymer having a very high viscosity at molten state and has a high glass transition temperature, it has been found that a cellulose ester film produced by melting a cellulose ester and extruding the melt from a die to be cast on a cooling drum or on a cooling belt may have problems that (i) leveling of the film is not fully easy; and (ii) the optical or mechanical property of the film is sometimes inferior to that of a cellulose ester film produced by a solution casting method (for example, refer to Patent Documents 1 and 2).
For example, a polarizer protective having a retardation function has been disclosed, which was produced by stretching a cellulose derivative film instead of a TAC film to obtain a retardation function, followed by being saponified and then being laminated with a polyvinyl alcohol polarizer (for example, refer to Patent Documents 3). However, when a melt cast cellulose ester film is used as a polarizer protective film of this type, the variation of retardation values due to a humidity change tends to be large, resulting in a practical disadvantage such as uneven contrast.
Therefore, it has been found that, when a melt cast film is stored for a long period in a state of being wound on a core, there is a problem of easy generation of horseback defects, and defects called as a “core set” and wrinkles in the film at the start of winding of a film master roll. In recent years, a film master roll is desired to have a wider width and a longer length of film in accordance with the popularization of a larger image screen. Therefore, there is a tendency that the width of a film master roll becomes wider and the weight of a film master roll becomes heavier, and this situation easily causes the above defects. Accordingly, an improvement is strongly desired.    Patent Document 1 Japanese Translation of PCT International-Application Publication No. 6-501040    Patent Document 2 Japanese Patent Application Publication, hereinafter, referred to as JP-A) No. 2000-352620    Patent Document 3 JP-A No. 2003-270442